Analysis of oligomeric protein complexes in the chloroplast sub‐proteome of nucleic acid‐binding proteins from mustard reveals potential redox regulators of plastid gene expression

Photosynthetic light quality acclimation in plants involves redox-controlled changes in plastid gene expression. To study proteins potentially involved in this regulation, we isolated low-abundant chloroplast nucleic acid-binding proteins from the crucifere mustard (Sinapis alba) and investigated if photosynthetic redox signals affect their composition and/or oligomeric structure. We purified chloroplasts from plants subjected to light quality shifts and applied organelle lysates to heparin-Sepharose chromatography followed by 2-D blue native PAGE. We studied accumulation and structure of oligomeric protein complexes and applied MS/MS to identify them. We found ten oligomeric protein complexes of higher order and eleven smaller protein complexes or spots including plastid-encoded RNA polymerase (PEP), plastid transcriptionally active chromosome proteins, RNA-binding proteins, ribosomal subunits and chaperones. A translation elongation factor was found to be the only protein displaying major differences in its amounts in response to the growth lights. Furthermore, we found a novel thioredoxin as a subunit of the PEP, a 2-Cys-peroxiredoxin complex and a (soluble) ferredoxin:NADP-oxido-reductase, which represent potential redox regulator of plastid gene expression. A T-DNA knock-out line of the thioredoxin from Arabidopsis exhibits a yellowish-pale phenotype, demonstrating that this novel PEP subunit is essential for proper plastid development.